IMDs for producing a therapeutic result in a patient are well known. Examples of such IMDs include, but are not limited to, implantable drug infusion pumps, implantable neurostimulators, cardioverters, cardiac pacemakers, defibrillators and cochlear implants. Such IMDs may treat a variety of symptoms or conditions including, but not limited to, chronic pain, migraine headaches, tremor, Parkinson's disease, epilepsy, incontinence, gastroparesis, heart failure, tachycardia, and bradycardia.
A common element in all of these IMDs is the need for electrical power in the device. The IMD requires electrical power to perform its function, which may include driving an electrical infusion pump, providing an electrical neurostimulation pulse, recording signals and/or providing an electrical cardiac stimulation pulse, for example.
Typically, a power source for an IMD can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power source for therapy is a large inconvenience.
A second type of power source utilizes primary cell batteries as the energy source of the IMD. This can be effective for low-power applications, such as cardiac pacing devices. However, such primary cell batteries usually do not supply the lasting power required to perform more energy-intensive functions. In some cases, such as involving an implantable artificial heart, a primary cell battery might last the patient only a few hours. In less extreme cases, a primary cell unit might expel its energy in less than a year. This is not desirable due to the need to explant and re-implant the IMD or a portion of the device.
One way to address the aforementioned limitations involves transcutaneously transferring electrical power through the use of inductive coupling. Such electrical power may then be optionally stored in a rechargeable (secondary) battery or capacitive element. Such a rechargeable battery may be used for directly powering the IMD. When the battery has mostly or totally expended its capacity, the battery may be recharged. This is accomplished transcutaneously using electromagnetic coupling from an external power source that is temporarily positioned on the surface of the skin. Most often this will involve inductive coupling, but could include other types of electromagnetic coupling such as RF coupling.
Transcutaneous energy transfer through the use of electromagnetic coupling generally involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. An internal, or “secondary” coil may be part of or otherwise electrically associated with the IMD. An external, or “primary” coil is associated with the external power source or recharging device. According to one method, the recharging device drives the primary coil with an alternating current. This induces a current in the secondary coil through inductive coupling. This current may then be used to power the IMD and/or to recharge an internal power source. Alternatively, RF coupling between the primary and secondary coils may be utilized for this purpose.
For IMDs, the efficiency at which energy is transcutaneously transferred is crucial for several reasons. First, the inductive coupling has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. By increasing the efficiency of the energy transfer between the primary and secondary coils, heating of the tissue is minimized. Moreover, the time required to complete the recharge session is minimized, thereby maximizing patient convenience. Additionally, by allowing the transfer of more energy during a shorter period of time, IMDs may be employed that have higher power requirements and that provide greater therapeutic and other advantages to the patient. Therefore, techniques for maximizing the efficiency at which energy is transcutaneously transferred are needed.